The present invention relates to a method and apparatus for indicating the air content of fresh concrete in situ.
It is common practice to entrain concrete mixes with air in order to resist freeze-thaw cycles and also to improve durability and workability. Since the quantity of entrained air is subject to variation due to a number of factors, and since these variations in air content affect the strength and durability of the concrete, it is important that the air content of a concrete mix be known.
A commonly used method for determining air content involves placing a sample of the concrete mix in a container of known volume, sealing the container, connecting with a pressurized chamber of known pressure and volume, allowing equalization of pressure from the pressurized chamber to the container and measuring the resulting pressure. The resulting pressure drop will be a function to the air content of the container in accordance with Boyle's law.
Although the aforesaid method can accurately determine the air content of a particular sample of concrete mixes, it does not necessarily indicate the air content of the concrete mix placed in situ. Transport and/or placing of the concrete mix subsequent to testing of a sample can reduce the air content significantly, due, for example, to overvibration, lag time and temperature effects. With present testing methods, such a reduction in air content may go undetected until premature deterioration occurs.